Cancer is a manifestation of neoplasia resulting from deviations in normal cell growth control. Errant production of growth factors or deviant signal cascades are typical examples of deregulation of cell growth control in tumor cells. Cell cycle progression is controlled by the successive formation, activation, and inhibition of complexes composed of cyclins and cyclin-dependent kinases (CDKs). Genomic mutations may cause unrestricted cellular proliferation by deregulating the cell cycle. The tumor suppressor p53 activates the expression of the CDK inhibitor p21CIP1, a negative cell cycle regulator. Like p21CIP1, p16Ink4A is a CDK inhibitor which converts active CDKs to inactive complexes. Human tumors are associated with mutations in the p16Ink4A family. In contrast, most cyclins promote cellular proliferation by activating CDKs. Several mutations affecting various cyclin genes have been implicated in carcinogenesis. The PI has cloned the human and mouse homologs of the novel cyclins G2 and G1. Both are highly expressed in various terminally differentiated tissues and, in contrast to classic cyclins, they are upregulated during responses to DNA damage and cell cycle arrest in lymphocytes. They have been implicated in cell cycle control, growth arrest, and the maintenance of the quiescent GO state of cells in differentiated tissues. The long- term goal of this work is to determine their role in these processes. The hypothesis that these cyclins act as negative coordinators of cell cycle progression will be tested. Specific Aim 1 is to characterize cyclin G2 and G1 protein complexes by immunoprecipitation and immunoblotting, employing cyclin G2- and G1-specific antibodies. The interaction with likely binding partners, such as CDKs or CDK-like kinases and B subunits of protein phosphatase 2A, will be evaluated. Functional aspects of these interactions will be investigated using (de)phosphorylation experiments. The subcellular localization of cyclin G2 and G1 will be examined during the cell cycle in B cell lines and in terminally differentiated tissues with immunohistochemical methods. Specific Aim 2 is to study their role in cellular growth inhibition by their induced overexpression in B cell lines. Since cyclin G2 and G1 are strongly expressed in brain cortex, they will also be overexpressed in neuronal cell lines, such as PC12, to define their role during differentiation. The proposed experiments will significantly contribute to the understanding of cyclin G2 and G1 function in the control of cellular proliferation under normal as well as pathological conditions. These studies will help to define processes gone awry in carcinogenesis or overproliferative immune responses, such as inflammatory and autoimmune diseases.